CTV network architectures for NTSC systems in the United States are fairly well established. Multiple system operators (MSOs) have networks which pass approximately 90% of the population with approximately 60-65% of the households actually being connected. These networks in the future will be upgraded to have more information carrying capacity and to better provide an increasing number of subscriber services.
In general, present CTV networks are broadband communications networks of coaxial cable and optical fiber which carry a plurality of 6 MHz amplitude modulated video channels on a frequency division multiplexed basis. The bandwidth of the typical CTV system is from 50 MHz to 550 MHz, which could increase in the future to over 1 GHz when increased amounts of optical fiber are used. CTV networks are very advantageous for providing a broadband communications path from a single point (headend) to multiple distribution points (subscribers), but are substantially limited in their return path. One reason is that the component equipment of such networks, which include amplifiers and compensation networks, are adapted to pass only for forward spectrum frequencies. Another reason is the noise which propagates in the reverse band of a single point to multiple point distribution system such as a CTV network. Because in the reverse direction the CTV network appears as an inverted tree, noise is propagated from every distribution point back to a single point, the headend. All of the individual noise contributions collectively add together to produce a very noisy environment and a communications problem at the headend.
In the past, it has not been considered advantageous that CTV networks could be used for telephony signals, which are low bandwidth voice signals that require point to point distribution and simultaneous bi-directional communication. Modem telephone networks, on the other hand, while limited in bandwidth do have the switching capability to provide point to point communications.
The advent of pay-per-view services, interactive television applications, and the business of providing various data services to subscribers has brought about a change in thinking about CTV networks. While not every MSO has an installed system which provides for two way communications, most cable television equipment has recently been designed to allow for limited upstream transmissions (in the direction from the subscriber to the headend).
Practically all modern CTV networks provide a so-called split or two-way system having one spectrum of frequencies for downstream or forward transmissions, which typically is from over 50 MHz to 550 MHz, and a second spectrum for upstream or reverse transmissions, which includes at least a band from approximately 5 MHz to 30 MHz. Generally, this reverse band comprises cable television channels T7 (5.75-11.75 MHz), T8 (11.75-17.75 MHz), T9 (17.75-23.75 MHz), and T10 (23.75-29.75 MHz). These return path channels, each having a 6 MHz television channel bandwidth, may be used for a variety of purposes.
Whether the CTV system is a so-called "sub-split", "mid-split", or "high-split" system, the two way transmissions in all three types of split transmission systems typically involve upstream transmissions at least in the 5-30 MHz band. The carriage of telephone signals should be able to take into account this limited bandwidth in the reverse direction of typical cable systems so as to not disturb the considerable investment MSOs have already made in their networks or to require extensive rebuilds to provide additional telephone service.
Another problem encountered with carrying telephony signals over the CTV networks is that there can be up to several hundred thousand subscribers served from a single headend. The bandwidth required for this number of subscribers for telephony service could make it too expensive to add to an operating CTV network because of its displacement of other revenue producing services. Therefore, there needs to be an advantageous method of limiting the bandwidth of telephony service on CTV networks before such services can become a reality.
Some CTV networks have recently developed an advantageous architecture which allows for the muse of limited frequency ranges for specialized groups of subscribers. These networks, having what is termed a "fiber to the serving area" (FTSA) architecture, contemplate dividing the subscriber base of a CTV network into manageable serving areas of approximately 400-2500 subscribers. Each serving area is coupled in a star configuration to the headend of a CTV network by an optical communications path ending in a fiber node. The fiber node acts as the distribution point of a high quality broadband CTV signal which is subsequently connected to the serving area subscribers over a coaxial cable distribution sub-network of feeders and drops in each serving area. The broadband signal in the forward direction is identical for each serving area to provide the same subscriber service to all customers in the subscriber base. In the reverse direction, it envisions an independent spectrum of frequencies associated with the particular serving area which can be used for different purposes, or many times for the same purpose.
The FTSA architecture provides the advantage of multiplying the bandwidth of the reverse portions of the spectrum times the number of serving areas. These portions of the broadband spectrum of the system not used in the forward direction, including the T7-T10 channels in the reverse band, are essentially spaced division multiplexed, and different auxiliary services can be provided to each serving area.
A final problem in the carriage of telephony signals over a CTV network is the interface between the telephony network and the CTV network. CTV networks are essentially local area networks which receive program feeds by satellite or by optical links and are thus limited to a small geographic areas. To be useful in the carriage of telephony signals, a CTV network should be able to seamlessly interface to a telephony network at a point where it is commercially viable to carry such signals. It must also provide signals which can pass to other parts of the interconnected telephone systems without extensive modulation or protocol changes to thereby become part of the international telephone system.